Accessing list values

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Example

Python lists are zero-indexed, and act like arrays in other languages.

lst = [1, 2, 3, 4]
lst[0]  # 1
lst[1]  # 2

Attempting to access an index outside the bounds of the list will raise an IndexError.

lst[4]  # IndexError: list index out of range

Negative indices are interpreted as counting from the end of the list.

lst[-1]  # 4
lst[-2]  # 3
lst[-5]  # IndexError: list index out of range

This is functionally equivalent to

lst[len(lst)-1]  # 4

Lists allow to use slice notation as lst[start:end:step]. The output of the slice notation is a new list containing elements from index start to end-1. If options are omitted start defaults to beginning of list, end to end of list and step to 1:

lst[1:]      # [2, 3, 4]
lst[:3]      # [1, 2, 3]
lst[::2]     # [1, 3]
lst[::-1]    # [4, 3, 2, 1] 
lst[-1:0:-1] # [4, 3, 2]
lst[5:8]     # [] since starting index is greater than length of lst, returns empty list
lst[1:10]    # [2, 3, 4] same as omitting ending index

With this in mind, you can print a reversed version of the list by calling

lst[::-1]    # [4, 3, 2, 1]

When using step lengths of negative amounts, the starting index has to be greater than the ending index otherwise the result will be an empty list.

lst[3:1:-1] # [4, 3]

Using negative step indices are equivalent to the following code:

reversed(lst)[0:2] # 0 = 1 -1
                   # 2 = 3 -1

The indices used are 1 less than those used in negative indexing and are reversed.

Advanced slicing

When lists are sliced the __getitem__() method of the list object is called, with a slice object. Python has a builtin slice method to generate slice objects. We can use this to store a slice and reuse it later like so,

data = 'chandan purohit    22 2000'  #assuming data fields of fixed length 
name_slice = slice(0,19)
age_slice = slice(19,21)
salary_slice = slice(22,None)

#now we can have more readable slices
print(data[name_slice]) #chandan purohit     
print(data[age_slice]) #'22'
print(data[salary_slice]) #'2000'

This can be of great use by providing slicing functionality to our objects by overriding __getitem__ in our class.

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Contributors: 17
2017-01-04
Licensed under: CC-BY-SA

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